Arc-shield arrangement in a vacuum power circuit breaker

ABSTRACT

An arc-shield arrangement in a vacuum power circuit breaker which has an interrupter unit which includes a generally cylindrical insulating envelope closed at both ends by end plates, stationary and movable electrode rods projecting into the envelope through one of the end plates, the arrangement protecting the insulating envelope from being contaminated with metal vapor of the arc plasma produced when the movable electrode rod is disconnected from the stationary electrode rod. To permit reduction of the overall dimensions of the insulating envelope, the arc-shield arrangement is adapted to form vacuum gaps in an axial direction of the envelope, in contrast to prior-art arcshield arrangements in which the gaps are formed annularly about the axis of a vacuum bulb.

5/1965 Titus 200/144 B I United States Patent [1 1 [111 3,9

Yanagisawa Nov. 18, 1975 ARC-SHIELD ARRANGEMENT IN A Primary ExaminerRobert S. Macon VACUUM POWER CIRCUIT BREAKER Attorney, Agent, or Firm-Haseltine, Lake & Waters [75] Inventor: Hifumi Yanagisawa, Tokyo, Japan 73 Assignee: Kabushiki Kaisha Meidensha, ABSTRACT Tokyo Japan An arc-shield arrangement in a vacuum power circuit [22] Filed; Apr, 3, 1974 breaker which has an interrupter unit which includes a generally cylindrical insulating envelope closed at [21] Appl' 4577l7 both ends by end plates, stationary and movable electrode rods projecting into the envelope through one of [30] Foreign Application Priority Data the end plates, the arrangement protecting the insulat- A 6, 1973 .l 4 mg envelope from bemg Contammated metal pr apan 8 41663 vapor of the arc plasma produced when the movable 52 Us. Cl 200/144 B etecttode ts disconnected the Stationary 51 int. c1. H01H 33/66 To Permit reduction 0f the Overall dhheh" [58] Field of Search 200/144 B sttths of the insulating envelope the arc-Shield rangement is adapted to form vacuum gaps in an axial [56] References Cited direction of the envelope, in contrast to prior-art arcshield arrangements in which the gaps are formed an- UNITED STATES PATENTS nularly about the axis of a vacuum bulb.

6 Claims, 3 Drawing Figures US. Patent Nov. 18, 1975 NfoFm ARC-SHIELD ARRANGEMENT IN A VACUUM POWER CIRCUIT BREAKER The present invention relates to power circuit breakers and, more particularly, to vacuum power circuit breakers, and arc-shield arrangements therein.

Vacuum power circuit breakers are principally used for polyphase medium-voltage power distribution purposes and generally comprise vacuum-bulb power interrupter units which are respectively allocated to individual phases of the current to be cut off. Each of the vacuum-bulb power interrupter units has a vacuum bulb which is composed of a generally cylindrical insulating envelope usually of glass or ceramics and a pair of metal end plates which are connected to longitudinal ends of the insulating envelope through metal-toinsulation seal elements. Stationary and movable electrode rods axially project into the vacuum bulb through the metal end plates and carry electrical contact elements at their respective leading ends which are located in the neighborhood of an axial center of the vacuum bulb.

When the movable electrode rod is axially moved toward the stationary electrode rod, the electrical contact elements carried on the two electrode rods are brought into contact with each other so that electrical connection is established between the electrode rods. When, conversely, the movable electrode rod is axially moved away from the stationary electrode rod, then the electrical contact elements on the two electrode rods are spaced apart from each other, forming an arcing region between the contact elements. An arc plasma is consequently produced in the arcing region with metal vapor emitted from the cathode electrode. The ions and metal particles forming the arc plasma are rapidly dispersed around the arcing region and disappear as the arc plasma is weakened in the vicinity of a current zero point so that high vacuum is restored in the arcing region, and accordingly the flow of current through the arcing region is interrupted.

In order to improve the insulation restoration characteristics of the vacuum power circuit breaker as a whole and to protect the insulating envelope from being stained with the metal particles forming the arc plasma, the vacuum-bulb power interrupter unit is usually provided with an arc-shield arrangement which consists of one or more generally cylindrical or ringshaped arc-shield elements which are adapted to cool, condense and thereby capture the metal particles dispersed around the arcing region between the spaced contact elements on the stationary and movable electrode rods.

Where only one arc-shield element is provided in the interrupter unit, the arc-shield element is usually positioned in such a manner that the arc-shield element concentrically surrounds the arcing region and thus forms an annular gap between each of the contact elements and that portion of the arcshield element which is in axially overlapping but radially spaced relation to the contact element.

Since, in this instance, only one annular gap is formed between the arc-shield element and each of the contact elements, there is a tendency that the electrical potential on the arc-shield element differs from the potential occurring at the center of the arcing region and assumes a value near to the potential on either of the contact elements.

Since, moreover, the spread of the metal vapour tends to be localized toward either of the contact elements immediately after the vapour has been produced between the contact elements, the electrical potential on the arc-shield element is subject to fluctuation in a broad range. All these result in unstable voltage characteristics of the circuit breaker and will thus invite restrikes of the are between the contact elements although they" have been disconnected from each other.

For the purpose of obviating these problems which result from the use of a single arc-shield element, an advanced version of arc-shield arrangement uses a plurality of, usually, two kinds of and three pieces of arcshield elements. The arc-shield elements of such an arrangement consist of a generally cylindrical main arcshield element which is in concentrically surrounding relation to the arcing region between the contact elements and a pair of generally ring-shaped auxiliary arcshield elements which extend axially inwardly from the end plates of the vacuum bulb.

The two auxiliary arc-shield elements have inner end portions which are in axially overlapping and radially spaced relation to axial outer end portions of the main arc-shield element so that an annular gap is formed between each of the auxiliary arc-shield elements and the overlapping outer end portion of the main arc-shield element. The ions and metal particles of the arc plasma produced as a result of the disconnection between the contacts elements are, thus, intercepted in a major proportion by the main arc-shield element but the remaining portion of the arc plasma is allowed to escape from the coverage of the main arc-shield element toward the inner faces of the end plates of the vacuum bulb. The

arc plasma allowed out of the main arc-shield element impinges upon the inner faces of the end plates and is caused to spread toward inner peripheral surfaces of the auxiliary arc-shield elements which are respectively located adjacent to the end plates.

In order that the vapor of the ions and metal particles thus spreading away from the end plates of the vacuum bulb be reliably and efficiently intercepted by the arcshield elements, it is of importance that each of the auxiliary arc-shield elements be in axially overlapping and radially spaced relation over a sufficient area to the adjacent outer end portion of the main arc-shield element so that an annular gap having an ample space is formed between the main arc-shield element and each auxiliary arc-shield element. Formation of such a gap apparently results in increased axial length and diameter of the insulating envelope and accordingly in an enlarged overall construction and an increased production cost of the interrupter unit as a whole.

The present invention contemplates the elimination of all these drawbacks of the prior art vacuum power circuit breakers and it is, accordingly, an important object of the present invention to provide an arc-shield arrangement in a vacuum power circuit breaker that contributes to the reduction of the diameter and the axial length of the vacuum bulb and accordingly to the reduction of the production cost of the circuit breaker.

It is another important object of the present invention to provide a vacuum power circuit breaker which has excellent voltage characteristics.

In accordance with the present invention, these and other objects are accomplished in a vacuum power circuit breaker which comprises the inventive arc-shield arrangement, wherein the circuit breaker includes a vacuum-bulb power interrupter unit including a generally cylindrical insulating envelope and a pair of end plates connected to axial ends of the insulating envelope through respective seal elements, a stationary electrode rod axially projecting into the envelope through one of the end plates of the bulb, a movable electrode rod axially projecting into the insulating envelope through the other of the end plates and axially movable toward and away from the stationary electrode rod.

The, stationary and movable electrode rods extend substantially in line with each other and carry electrical contact elements at their respective leading end portions for providing an arcing region between the contact elements when the movable electrode rod is axially moved away from the stationary electrode rod.

The inventive arc-shield arrangement is characterized by a main arc shield element having an axially central cylindrical portion which is in concentrically surrounding relationship to and radially spaced apart from the arcing region between the two contact elements and axially outer end portions which extend axially outwardly from the central cylindrical portion and are respectively in concentrically surrounding relationship to leading end portions of the electrode rods.

There is a pair of auxiliary arc-shield elements respectively mounted on the inner faces of the end plates, each arc-shield element having a generally cylindrical portion projecting axially inwardly from the inner face of the associated end plate and extending in proximity to and parallel with the inner peripheral surface of the seal element adjacent to the end plate.

. There is furthermore an annular portion extending radially inwardly and axially toward the inner face of the associated end plate from an axially inner end of the cylindrical portion of the auxiliary arc-shield element, each of the latter being axially spaced apart from each of the axial ends of the main arc-shield element, and forming an axial gap between each axial end main arc-shield element and each of the arc-shield elements.

The axially outer end portions of the main arc-shield element may be made substantially smaller in diameter than the axially central portion of the main arc-shield element. To impede the formation of sparks around the axially outer end portions of the main arc-shield element, each axially outer end portion of the main arcshield element may have a circumferential edge which is rounded into a hoop form.

The features and advantages of the vacuum power circuit breaker according to the present invention over the prior-art counterparts will be better understood from the following description taken in conjunction with the accompanying drawings, in which like reference numerals and characters designate corresponding units, members and portions, and in which:

FIG. 1 is a longitudinal'sectional view which sown an example of a prior-art vacuum-bulb power interrupter unit of a vacuum circuit breaker, using a plurality of arc-shield elements;

FIG. 2 is a view similar to FIG. 1 showing another example of the prior-art vacuum-bulb power interrupter unit, using a plurality of arc-shield elements; and

FIG. 3 is a longitudinal sectional view which schematically illustrates a preferred embodiment of a vacuum-bulb power interrupter unit according to the present invention.

Reference will now be made to the drawings, first to FIG. 1 so as to more clearly point out the drawbacks inherent in a prior-art vacuum-bulb power interrupter unit incorporating an arc-shield arrangement which consists of two kinds of and three pieces of arc-shield elements.

As shown in FIG. 1, a vacuum-bulb power interrupter unit of a known vacuum circuit breaker includes a vacuum bulb which is composed of a generally cylindrical insulating envelope l and a pair of, upper and lower, metal end plates 12 and 14 to which the insulating envelope is securely and hermetically connected at its axial ends through metal-to-insulation seal elements 16 and 18, respectively, which form part of the insulating envelope 10. The insulating envelope 10 is herein shown as comprising a pair of cylindrical members 20 and 22 of usually glass or ceramics. The two cylindrical members 20 and 22 are connected at their axially outer ends to the upper and lower end plates 12 and 14 through the seal elements 16 and 18, respectively, and at their axially inner ends to each other through a ringshaped connecting member 24.

A stationary electrode rod 26 axially projects into the vacuum bulb through the upper end plate 12 and carries at its leading end an electrical contact element 28. The stationary electrode rod 26 is fixed to the vacuum bulb or otherwise to a suitable stationary member or structure which is independent of the vacuum bulb and is electrically connected to a leading-in line of the vacuum power circuit breaker.

The lower end plate 14 of the vacuum bulb is formed with a central aperture 14a through which a movable electrode rod 30 axially projects into the vacuum bulb. The movable electrode rod 30 extends in line with the stationary electrode rod 26 and carries at its leading end an electrical contact element 32 similarly to the stationary electrode rod 26. The movable electrode rod 30 is electrically connected to a leading-out line of the vacuum circuit breaker and is mechanically connected to an actuating member of a control mechanism which is usually located below the interrupter unit, though not shown. The movable electrode rod 30 is thus driven by the control mechanism to axially move toward and away from the stationary electrode rod 26 so that electrical connection is established or interrupted between the contact elements 28 and 32 on the stationary and movable electrode rods 26 and 28, respectively. The central aperture 14a in the lower end plate 14 of the vacuum bulb is sealed off by means of a metallic bellows assembly 34 which is connected between the end plate 14 and the movable electrode rod 30.

When the movable electrode rod 30 is axially moved toward the stationary electrode rod 26, the contact element 32 on the movable electrode rod 30 is brought into contact with the contact element 32 on the stationary electrode rod 26 so that electrical connection is established between the two electrode rods 26 and 30. When, conversely, the movable electrode rod 30 is axially moved away from the stationary electrode rod 26, then the contact element 32 on the former is spaced apart from the contact element 28 on the latter so that an arcing region 36 is provided between the spaced contact elements 28 and 32.

To protect the insulating envelope 10 from being contaminated with the metal vapour of the arc thus produced, the vacuum-bulb power interrupter unit has incorporated into the vacuum bulb an arc-shield arrangement which is shown to comprise a main arcshield element 38 supported by the connecting member 24 through an annular support member 39 and a pair of cylindrical auxiliary arc-shield elements 42 and 44 which are supported by the upper and lower end plates 12 and 14, respectively, of the vacuum bulb.

The main arc-shield element 38 has an axially intermediate portion 380 which is in concentrically surrounding and radially spaced relation to the arcing re gion 36 or, in other words, to the contact elements 28 and 32 in the spaced apart conditions shown and upper and lower end portions 38b and 380, respectively, which extend axially outwardly from the intermediate portions 38a and which are in concentrically surrounding and radially spaced relation to inner or leading end portions of the stationary and movable electrode rods 26 and 30, respectively. The intermediate portion 380 of the main arc-shield element 38 is radially in proximity to the inner surface of the insulating envelope whereas the upper and lower end portions 38b and 380 of the main arc-shield element 38 have reduced diameters and are thus radially inwardly spaced apart from the inner surface of the insulating envelope 10.

On the other hand, the upper and lower auxiliary arcshield elements 40 and 42 extend axially inwardly from the inner faces of the upper and lower end plates 12 and 14, respectively, and'have axially inner end portions which are in concentrically surrounding and radially spaced relation to the reduced upper and lower end portions 38b and 380, respectively. An annular gap 44 is thus formed between the upper end portion 38b of the main arc-shield element 38 and the lower end portion of the upper auxiliary arc-shield element 40 and, likewise, an annular gap 46 is fonned between the lower end portion 38c of the main arc-shield element 38 and the upper end portion of the lower auxiliary arcshield element 42, as illustrated. Designated by reference numeral 48 is a generally cupshaped bellows shield element which is supported by the movable electrode rod for the purpose of protecting the bellows assembly 34 from are plasma.

trode rod 26 is intercepted by the main arc-shield element 38 so that the metal particles of the plasma are largely captured by the main arc-shield element 38.

The remaining portion of the arc plasma is, however,

not captured by the main arc-shield element 38 and is thus allowed out of the main arc-shield element 38 toward the upper and lower end plates 12 and 14 throughthe open axial ends of the main arc-shield element 38. I

The metal particles of arc plasma thus allowed out of the mainarc-shield element 38 impinge upon the inner faces of the upper and lower end plates 12 and 14 and are partly re-directed toward the annular gaps 44 and 46 between the upper and lower end portions 38b and 38c of the main arc-shield element 38 and the axially overlapping portions of the upper and'lower auxiliary arc-shield elements and 42. The insulating envelope 10 is in this manner shielded from the arc plasma primarily by the main arc-shield element 38 and secondarily by the auxiliary arc-shield elements 40 and 42. In order that the insulating envelope 10 be reliably isolated from the arc plasma, it is important that the annular gaps 44 and 46 have sufficient axial lengths which will give rise to an increase in the axial length of the vacuum bulb.

Since, moreover, the gaps 44 and 46 are formed annularly about an axis of the vacuum bulb, the insulating envelope 10 should have a diameter which is large enough to provide sufficient radial spacings between the main arc-shield element 38 and the auxiliary arcshield elements 40 and 42. The thus increased axial length and diameter of the vacuum bulb results in an enlarged unwieldy overall construction and an increased production cost of the vacuum-bulb power interrupter unit is previously noted.

These drawbacks are to some extent alleviated in an arc-shield arrangement incorporated into the vacuumbulb power interrupter unit which is illustrated in FIG. 2. Referring to FIG. 2, this advanced version of a priorart arc-shield arrangement comprises, in addition to the bellows shield element 34, a main arcshield element 50 which is supported by the connecting member 24 of the insulating envelope 10 through an annular support member 52 and a pair of auxiliary arc-shield elements 54 and 56 supported by the upper and lower end plates 12 and 14, respectively, of the vacuum bulb.

The main arc-shield element 50 has an axially intermediate portion 50a which is in concentrically surrounding and radially spaced relation to the arcing region 36, viz., to the electrical contact elements 28 and 32 in the spaced apart conditions and upper and lower end portions 50b and 50c which extend axially outwardly and radially inwardly from the intermediate portion 50a and which are in concentrically surrounding and radially spaced relation to the inner or leading end portions of the stationary and movable electrode rods 26 and 30, respectively. To impede the production of sparks around the open ends of the main arc-shield 50, the upper and lower end portions 50b and 500 of the main arc-shield 50 have circumferential edges 50d and 50e, respectively, each of which is rounded into a hoop form as illustrated.

On the other hand, the auxiliary arc-shield elements 54 and 56 have generally arcuate cross sections and are secured at their top and bottom to the inner faces of the upper and lower end plates 12 and 14, respectively. The upper and lower auxiliary arc-shield elements 54 and 56 thus have respective inner faces which are concave to the adjacent axial ends of the main arc-shield element 50.

The auxiliary arc-shield elements 54 and 56 have skirt portions 54a and 56b, respectively, which are also rounded into loop forms so as to prevent the formation of sparks therearound. These rounded skirt portions 54a and 56b of the upper and lower auxiliary arc-shield elements 54and 56, respectively, are in concentrically surrounding and radially outwardly spaced relation to the rounded edge portions 50d and 50e of the main arcshield element 50. An annular gap 58 is thus formed between the rounded upper edge portion 50d of the main arc-shield element 50 and the rounded skirt portion 54a of the upper auxiliary arc-shield element 54 and, likewise, an annular gap 60 is formed between the rounded lower edge portion 502 of the main arc-shield element 50 and the rounded skirt portion 56a of the lower auxiliary arc-shield element 56, as shown.,

Since,-thus, the gaps 58 and 60 are formed annularly about the axis of the vacuum bulb, the main and auxiliary arc-shield elements should have sufficient axial lengths and, besides, the auxiliary arc-shield elements should have sufficient diameters for providing ample spaces to accomodate the gaps 58 and 60. The drawbacks inherent in the interrupter unit illustrated in FIG. 1 are therefore also encountered in the interrupter unit shown in FIG. 2 although the latter may have an axial length which is shorter relative to the diameter than that of the former. The gist of the present invention consists in the provision of an axial gap, in lieu of the annular gap, between the main arc-shield element and each of the auxiliary arc-shield elements so as to permit significant reduction of the diameter and the axial length of the interrupter unit. 7

Referring to FIG. 3, the inventive arc-shield arrangement herein proposed is shown as being incorporated into a vacuum bulb which in itself is similar in construction to a prior-art vacuum bulb, which may be constructed as illustrated in FIGS. 1 or 2. The description made in respect of the construction of the vacuum-bulb power interrupter unit illustrated in FIG. 1 thusapplies as it is'to the embodiment of FIG. 3 except for the arcshield arrangement incorporated therein. it is, however, to be noted that the arc-shield arrangement of the nature herein proposed may be applied to a vacuumbulb power interrupter having any other construction.

As seen in FIG. 3, the novel arc-shield arrangement, which forms part of the vacuum-bulb power interrupter unit of the vacuum circuit breaker, embodying the present invention comprises a main arc-shield element 62 which is supported by the connecting member 24 of the insulating envelope 10 through an annular support member 64 and a pair of, upper-and lower, auxiliary rectly by the upper and lower metal end plates 12 and 14, respectively, of the vacuum bulb.

The main arc-shield element 62 has a generally cylindrical, axially intermediate portion 62a and upper and lower end portions 62b and 620 extending axially outwardly and radially inwardly from the intermediate portion 62a. The cylindrical intermediate portion 620 of the main arc-shield element 62 is in concentrically surrounding and radially outwardly spaced relation to thearcing gap 36 or, in other words, to the electrical contacts 28 and 32, in axially spaced conditions, on the stationary and movable electrode rods 26 and 30, re-

spectively. On the other hand, the upper and lower end portions 62b and 620 of the main arc-shield element 62 are in concentrically surrounding and radially spaced relation to inner or leading end portions of the stationary and movable electrode rods 26 and 30.

The lower end portion 62(- is also in concentrically surrounding relation to the bellows shield element 48 which contains therein part of the bellows assembly 34.

While the intermediate portion 62a of the main arcshield element 62 is located radially in proximity to the insulating envelope 10, the upper and lower end portions 62b and 62c of the arc-shield element 62 have reduced diameters and are thus radially spaced apart from the insulating envelope. 10 as shown.

The upper and lower end portions 62b and 62c'of the mainarc-shield element 62 are, moreover, shown to have circumferential edges 62d and 62e, respectively, each of which is rounded into a hoop form so as to impede production of sparks from-the edge. The upper auxiliary arc-shield element 66 has a cylindrical portion 660 extending axially inwardly from the inner face of the upper end plate 12 of the vacuum bulb and radially inclose proximity to the metal-to-insulation seal element 16 between the upper and plate 12 and the insulating envelope l0 and having an axial length which is appreciably larger than an axial length of the seal element 16. r

The upper auxiliary arc-shield element 66 further has an annular portion 66b extending radially inwardly and axially outwardly from the cylindrical portion 66a and having an inner peripheral edge which is in concentrically surrounding and radially spaced relation to the stationary electrode rod 26. The annular portion 66b of the upper auxiliary arc-shield element 66 thus has an inner circumferential edge 66c encircling the stationary electrode rod 26 and a concave inner face confronting the upper end of the main arc-shield element 62. A gap 70 is thus formed axially between the upper end of the main arc-shield element 62 and the upper auxiliary arcshield element 66. The lower auxiliary arc-shield element 68 is shaped similar to the upper auxiliary arcshield element 66, having a generally cylindrical portion 68a extending axially inwardly from the lower end plate 14 and extending close to and appreciably longer than the seal element 18 and an annular portion 68b extending radially I arc-shield elements 66 and 68 which are supported diinwardly and axiallyoutwardly from the cylindrical 68c encircling the movable electrode rod 30 and accordingly the bellows assembly 34 and a concave inner face which confronts the lower end of the main arcshield element 62. A gap 72 is thus formed axially between the lower end of the main arc-shield element 62 and the lower auxiliary arc-shield element 68.

Theions and metal particles of arc plasma produced in the arcing region 36 when the movable electrode rod 30 is moved away from the stationary electrode 26 are captured in a major proportion by the main arc-shield element 62. The remaining portion of the arc plasma which is allowed out of the main arc-shield element 62 through the upper and lower openends thereof is partly admitted into annular spaces between the upper and lower end plates 12 and 14 and the annular portions 66b and 68b of the upper and lower auxiliary arc-shield elements 66 and 68, respectively, through the central apertures defined by the inner circumferential edges 66c yand 680 and partly impinges upon the concave inner faces of the annular portions 66b and 68b so as to be captured in part by the auxiliary arc-shield elements 66 and 68 and in part re-directed from the concave surfaces of the elements backwardly into the space which is defined by the main arc-shield element 62.

While the annular portions 66b and 68b of the auxiliary arc-shield elements 66 and 68, respectively, are effective to capture or re-direct the arc plasma, the cylindrical portions 660 and 68a of the elements 66 and 68 serve to lessen the strength of the electric fields which tend to be concentrated around the metal-to-insulation seal elements 16 and 18.

main arc-shield element 62 and each of the upper and lower auxiliary arc-shield elements 66 and 68.

From the foregoing description, it will now be appreciated that the following advantages can be achieved in a vacuum power circuit breaker according to the present invention by virtue of the arc-shield arrangement of the described nature:

a. Because of the fact that the main arc-shield element 62 is not axially overlapping relation to the auxiliary arc-shield elements 66 and 68, the axial lengths of the main and auxiliary arc-shield elements and the diameters of the auxiliary arc-shield elements can be significantly reduced so that providing small-sized overall construction of the vacuum bulb. As a matter of fact, it has been asertained that the diameter of the vacuum bulb of the construction shown in FIG. 3 can be reduced approximately per cent from that of the vacuum bulb incorporating the arc-shield arrangement of, for example, the nature illustrated in FIG. 1 or 2.

b. In spite of the fact that the gaps providing the necessary withstand voltages between the main and auxiliary arc-shield elements are formed in the axial direction of the vacuum bulb, the axial length of the vacuum bulb need not be increased but, contrariwise, can be considerably reduced. This is because of the fact that each of the auxiliary arc-shield elements has a portion located in proximity to the seal element and a portion effective to re-direct the arc plasma into the main arcshield element so that the field strength to which the seal element is to be subjected can be effectively alleviated.

c. By virtue of the significant reduction of the axial lengths of the main and auxiliary arc-shield elements and the diameter of the auxiliary arc-shield elements, the costs for the production and installation of the vacuum power circuit breaker can be reduced to a considerable degree.

While only one preferred embodiment of the arcshield arrangement, in a vacuum power circuit breaker, according to the present invention has been described and shown, such is merely for the purpose of illustration of the essential features of the present invention and is therefore subject to various changes and modifications where desired.

What is claimed is:

1. An arc-shield arrangement in a vacuum power circuit breaker which includes a vacuum-bulb power inable toward and away from said stationary electrode rod, said electrode rods extending substantially in line with each other and carrying electrical contact elements at their respective leading end portions for providing an arcing region between said contact elements when said movable electrode rod is axially moved away from said stationary electrode rod, the arrangement comprising a main arc-shield element having an axially central cylindrical portion which is in concentrically surrounding relationship to and radially spaced apart from said arcing region and axially outer end portions which extend axially outwardly from said central cylindrical portion and are respectively in concentrically surrounding relationship to said leading end portions of the electrode rods, and a pair of auxiliary arc-shield elements respectively mounted on the inner faces of said end plates, said auxiliary arc-shield elements having generally cylindrical portions projecting axially inwardly from said inner faces of the associated end plates and extending in proximity to and parallel with the inner peripheral surfaces of said seal elements adjacent to said end plates, annular portions extending radially inwardly and axially toward said inner faces of the associated end plates from axially inner ends of said cylindrical portions of the auxiliary arc-shield elements, the latter being axially spaced apart from the respective axial ends ofsaid main arc-shield element, and forming an axial air gap between said axial ends of the main arcshield element and said auxiliary arc-shield elements.

2. The arc-shield arrangement as defined in claim 1, wherein said annular portions of the auxiliary arcshield elements have concave faces confronting said axial ends of the main arc-shield element.

3. The arc-shield arrangement as defined in claim 2, wherein said annular portions of the auxiliary arcshield elements are formed with apertures defined by inner edges encircling that portion of one of said electrode rods which is close to said inner faces of the end plates on which a respective one of said auxiliary arcshield elements is mounted.

4. The arc-shield arrangement as defined in claim 3, wherein saidcylindrical portions of the auxiliary arcshield elements extend beyond the axially inner ends of the respective seal elements adjacent thereto.

5. The arc-shield arrangements as defined in claim 4, wherein said outer end portions of the main arc-shield element are smaller in diameter than said central cylindrical portion thereof.

6. The arc-shield arrangement as defined in claim 5, wherein said outer end portions of the main arc-shield element have radially inwardly rolled circumferential ends. 

1. An arc-shield arrangement in a vacuum power circuit breaker which includes a vacuum-bulb power interrupter unit including a generally cylindrical insulating envelope and a pair of end plates connected to axial ends of said envelope through respective seal elements, a stationary electrode rod axially projecting into said envelope through one of said end plates, and a movable electrode rod axially projecting into said envelope through the other of said end plates and axially movable toward and away from said stationary electRode rod, said electrode rods extending substantially in line with each other and carrying electrical contact elements at their respective leading end portions for providing an arcing region between said contact elements when said movable electrode rod is axially moved away from said stationary electrode rod, the arrangement comprising a main arc-shield element having an axially central cylindrical portion which is in concentrically surrounding relationship to and radially spaced apart from said arcing region and axially outer end portions which extend axially outwardly from said central cylindrical portion and are respectively in concentrically surrounding relationship to said leading end portions of the electrode rods, and a pair of auxiliary arcshield elements respectively mounted on the inner faces of said end plates, said auxiliary arc-shield elements having generally cylindrical portions projecting axially inwardly from said inner faces of the associated end plates and extending in proximity to and parallel with the inner peripheral surfaces of said seal elements adjacent to said end plates, annular portions extending radially inwardly and axially toward said inner faces of the associated end plates from axially inner ends of said cylindrical portions of the auxiliary arc-shield elements, the latter being axially spaced apart from the respective axial ends of said main arc-shield element, and forming an axial air gap between said axial ends of the main arc-shield element and said auxiliary arcshield elements.
 2. The arc-shield arrangement as defined in claim 1, wherein said annular portions of the auxiliary arc-shield elements have concave faces confronting said axial ends of the main arc-shield element.
 3. The arc-shield arrangement as defined in claim 2, wherein said annular portions of the auxiliary arc-shield elements are formed with apertures defined by inner edges encircling that portion of one of said electrode rods which is close to said inner faces of the end plates on which a respective one of said auxiliary arc-shield elements is mounted.
 4. The arc-shield arrangement as defined in claim 3, wherein said cylindrical portions of the auxiliary arc-shield elements extend beyond the axially inner ends of the respective seal elements adjacent thereto.
 5. The arc-shield arrangements as defined in claim 4, wherein said outer end portions of the main arc-shield element are smaller in diameter than said central cylindrical portion thereof.
 6. The arc-shield arrangement as defined in claim 5, wherein said outer end portions of the main arc-shield element have radially inwardly rolled circumferential ends. 